Field of the Invention
The present invention is directed generally to components used with medium voltage electrical power cables and, more particularly, to components used to inject a fluid into an interior of a cable.
Description of the Related Art
A known problem that occurs in power cables (e.g., medium voltage solid dielectric power cables in underground distribution networks) is the formation of concentrations of moisture, sometimes referred to as “water trees,” in the insulation that surrounds the cable conductor (e.g., twisted wire strands). This dielectric breakdown is generally attributed to a “treeing” phenomena (i.e., formation of oxidized polymer in dendritic patterns within the insulation material that resemble trees), which leads to a progressive degradation of the cable's insulation.
Treatment fluids (e.g., phenylmethyldialkoxysilane, dimethyldialkoxysilane, tolylethylmethyldialkoxysilane, cyanobutylmethyldialkoxysilane, and the like) have been developed that are injected into the interior of the cable, diffuse into the insulation, and interact with the moisture in the micro-voids. This process is sometimes referred to as cable rejuvenation. To inject the treatment fluid, an injection port must be installed that provides fluid communication with the interior of the cable. For example, U.S. Pat. Nos. 7,195,504 and 7,538,274 describe injection adapters suitable for Sustained Pressure injection of rejuvenation treatment fluid into a power cable. Sustained Pressure Rejuvenation (“SPR”) differs from earlier injection methods because the injection occurs at higher pressures, typically greater than 30 psi, and the pressure is sealed inside the cable, and sustained therein, when injection has been completed. Such SPR injection is generally performed on de-energized cables. However, SPR injection may be used on energized cables terminated at both ends by live-front terminators that allow physical fluid access to the interior of the cable.
There are times when it is desirable to introduce a treatment fluid into and withdraw a treatment fluid from an energized cable having at least one dead-front termination (e.g., when rejuvenating a cable with a dielectric enhancement fluid). This is typically done at dead-front terminations implemented using dead front injection elbows, such as those described by U.S. Pat. Nos. 4,946,393 and 6,332,785. But it can also be done at single piece injection splices and modular injection splices, which each have an injection port. Cable accessories that include an injection port are generally referred to hereinafter as “injection components.”
Unfortunately, currently available dead front injection components (e.g., dead front injection elbows and injection splices) used to introduce a restorative fluid into a cable's interior suffer from at least one or more of the following eight shortcomings.
First, because the treatment fluid comes into intimate contact with the entirety of the annular interior of the injection component, a portion of the treatment fluid is wasted. Injection components typically include a semi-conductive insert, a surrounding layer of insulation, and a semi-conductive exterior layer. Unfortunately, a significant wasted portion of the treatment fluid injected into the injection component permeates into the semi-conductive insert, the surrounding layer of insulation, and the semi-conductive exterior layer. Further, at least some of the wasted portion exits the injection component into the surrounding environment, and represents a significant fluid loss. Depending upon cable geometry, fluid delivery method, injection pressure, and operating temperature, this loss may range from about 5% to about 15% of the treatment fluid supplied to the injection component. Further, this loss could exceed 15%.
Second, the treatment fluid may cause subcomponents of the injection component to swell and exceed desired tolerances and/or fail. For example, the treatment fluid may cause ethylene propylene diene monomer (“EPDM”) rubber and ethylene propylene rubber (“EPR”), the most common polymers used in injection components, to swell in excess of 40%w at cable operating temperatures above about 50° C. This is a larger factor when a soak period is utilized (e.g., in small cables) to provide sufficient fluid to the interiors of the cables. An injection component experiencing such swelling will no longer meet industry standard dimensional requirements, such as those of IEEE 386™. Further, the treatment fluid may cause silicone rubber (often used to construct cable termination and splicing accessories) to swell in excess of 40%w at ambient temperatures of about 20° C. Swelling to these extents can lead to failure of the component.
Third, currently available injection components limit maximum injection pressures to a level that is less than optimum for cable rejuvenation. Cable accessories (e.g., elbows and splices) that have been designed to accommodate fluid injection rely on an interference fit between the cable accessory and the cable insulation to retain fluid pressure. Generally this interface cannot contain pressures in excess of 30 psi. On the other hand, testing has shown that cable insulation can withstand pressures up to 1000 psi (dependent on configuration and insulation material) and that using higher pressures improves the quality of the treatment. Bertini & Keitges, “Silicone Injection: Better with Pressure,” ICC, Sub. A., May 19, 2009.
Fourth, externally applied conventional hose clamps that compromise the electrical integrity of the injection component are required to operate the injection component at higher pressures. Currently utilized injectable components can withstand a maximum internal pressure within a range of about 5 psig to about 30 psig depending upon the size of the cable, the design of the injection component, operating temperature, and the materials used to construct the injection component. Often, to operate at the higher end of this range, an external hose clamp is applied to the injection component to counteract hoop stress caused by the fluid pressure. Unfortunately, the hose clamp deforms the injection component and compromises the electrical integrity of the injection component. Additionally, the hose clamps are typically left in place, and creep over time, which further compromises the electrical integrity of the injection component. While these hose clamps may be removed after the treatment is completed, doing so requires an additional visit to the cable termination, which increases both expense and risk of injury.
Fifth, a portion of the treatment fluid may leak from the branch of a treatment elbow that houses the probe pin. Injection elbows are the most common dead-front components used to inject treatment fluid into a cable. An O-ring or D-ring seal is conventionally applied to the base of the probe pin to prevent fluid from leaking out of the branch of the elbow housing the probe pin and into the environment or a mated bushing. Unfortunately, this seal has been known to leak, causing damage to bushings, and creating a fire or explosion hazard. This problem is described in Bertini & Brinton, “A Comparison of Rejuvenation Hazards,” EDIST 2009, Jan. 13, 2009, which is incorporated herein by reference in its entirety.
Sixth, whenever the injection port is open (e.g., an injection cap or a permanent cap has been removed) some of the treatment fluid may flow out through the open injection port. This decreases residual pressure in the cable and (proportionally) the volume of the treatment fluid in the cable. Treatment fluid may spray or dribble from the injection port and create a hazard potential for fire, injure personnel, and/or contaminate the environment.
Seventh, the permanent cap used to close the injection port of some types of injection components may be mistaken for a cap used to seal other types of devices found on cable accessories that are not used to inject treatment fluid into cables. For example, many permanent caps have an external ring-shaped attachment point that is used to remove and install the cap. This ring-shaped attachment point may be mistaken for the external ring-shaped attachment point of a cap used on other devices mounted on cable accessories. For example, the external ring-shaped attachment point of the permanent cap may be mistaken for an eye (or eyelet) included on an elbow and used to pull on the elbow. By way of another example, the external ring-shaped attachment point of the permanent cap may be mistaken for a similar structure on a cover used to close a capacitive test point that can easily be removed by a standard hot stick implement. Such mistakes can result in the permanent cap being removed from the injection port, which exposes the cable conductor directly to atmosphere, creates a passage through which foreign objects can come in contact with the voltage of the cable conductor, and a passage through which potential can spontaneously and violently flash-over creating an arc flash and a power outage. The temperature of an arc flash can reach 35,000° F. and hence poses a substantial threat to operators and nearby equipment. Personnel unfamiliar with the function of the injection port can expose themselves to danger, create a hazard for others, and initiate a failure point if the permanent cap is not promptly replaced and/or is handled improperly.
Therefore, a need exists for new injection components that avoid one or more of the shortcomings discussed above. The present application provides these and other advantages as will be apparent from the following detailed description and accompanying figures.